CABCHARGE vouchers that his former aide James Hunter Ashby alleges were handed over blank by Peter Slipper appear to have been filled out in the Speaker's handwriting.

Julia Gillard has vowed Labor will quickly restore Mr Slipper to the Speaker's chair if he is cleared of criminal allegations over the Cabcharge entitlements, challenging the Coalition to use parliamentary process to remove him while separate civil claims of sexual harassment are heard.

Julia Gillard has vowed Labor will quickly restore Mr Slipper to the Speaker's chair if he is cleared of criminal allegations over the cabcharge entitlements. Photo: Andrew Meares

The Department of Finance has sent Mr Slipper copies of vouchers for at least two of the three days Mr Ashby alleges they were handed over signed, but without any details filled in.

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They appear to be filled out in the same handwriting as each other, and in handwriting very similar to Mr Slipper's writing in the 2008 parliamentary pecuniary interest register. The dates on some of the copied vouchers are not legible.

''These documents have all been completed by me and are clearly in my handwriting as I said they were. The so-called criminal allegation is a complete fabrication, just as the other claims are not accurate,'' Mr Slipper said in a statement.

Ms Gillard's stance over Mr Slipper's return while the sexual harassment claims are outstanding sets the scene for a battle when Parliament returns on May 8, budget day. It effectively challenges the Opposition Leader, Tony Abbott, to try to use a parliamentary vote to remove Mr Slipper from the chair.

It comes as despairing Labor MPs have again begun questioning the viability of Ms Gillard's leadership and insisting that Labor act to remove the growing impression it would stoop to anything to cling to power.

Sources said cracks were also appearing among the majority element of the NSW Right which backed Ms Gillard against Kevin Rudd in the leadership ballot just two months ago.

Labor strategists had earlier told the Herald they were resigned to Mr Slipper not returning to the speakership for many months, after Tony Windsor backed fellow independent MP Andrew Wilkie and the Coalition in the view Mr Slipper should stand aside while civil and criminal matters were investigated.

But speaking to reporters in Turkey, Ms Gillard said "the best thing you can do in circumstances like this is to look to do what is right and based on the precedent of the Parliament, not to try and invent rules as you go along''.

"When we look to the history of the Parliament we can see a clear set of precedents where people have continued to function in their office while the subjects of civil matters,'' she said.

Mr Abbott said Ms Gillard ''seems to be the only one in the country who thinks that the Speaker can resume the chair prior to all the allegations against him being fully investigated''.

The Workplace Relations Minister, Bill Shorten, said ''sexual harassment is completely, completely unacceptable at any level''.

The backbencher Patrick Secker denied he was considering defecting from the Liberals to replace Mr Slipper as Speaker.

A spokesman for Mr Ashby said police ''should be allowed to continue their criminal investigation unimpeded by speculation''.